The present application relates to the magnetic resonance imaging arts. It finds particular application to a magnetic resonance imaging apparatus and method that efficiently provides depth information by reading out a selected number of planes in k-space, each plane of data representing a view or projection through a volume of interest, and will be described with particular reference thereto. However, the technique may also find application in other magnetic resonance imaging techniques.
Magnetic resonance imaging (MRI) is a noninvasive imaging technique that provides clinicians and diagnosticians with information about the anatomical structure and condition of a region of interest within a subject. Commonly, in MRI, a substantially uniform temporally constant main magnetic field (B0) is set up in an examination region in which a subject being imaged or examined is placed. Via radio frequency (RF) magnetic field (B1) excitation and manipulations, selected magnetic dipoles in the subject which are otherwise aligned with the main magnetic field are tipped to excite magnetic resonance. The resonance is typically manipulated to induce detectable magnetic resonance echoes from a selected region of the subject. In imaging, the echoes are spatially encoded via magnetic gradients set up in the main magnetic field. The raw data from the MRI scanner is collected into a matrix, commonly known as k-space. By employing inverse Fourier, two-dimensional Fourier, three-dimensional Fourier, or other known transformations, an image representation of the subject is reconstructed from the k-space data.
There are many applications in which depth or 3D information is useful for diagnosis and formulation of treatment strategies. For example, in imaging blood vessels, cross-sections merely show slices through vessels, making it difficult to diagnose stenosis or other abnormalities. Likewise, interventional imaging, such as needle tracking, catheter tracking, and the like, requires 3D information. Also, depth information is useful in the so-called interactive imaging techniques in which images are displayed in real or near-real time and in response to which the operator can adjust scanning parameters, such as view angle, contrast parameters, field of view, position, flip angle, repetition time, and resolution.
Three-dimensional imaging generally involves either acquiring multiple two-dimensional or slice images which are combined to produce a volumetric image or, alternately, the use of three-dimensional imaging techniques. Much effort at improving the efficiency of volume imaging has been focused on speeding up the acquisition. For example, many two-dimensional fast scan procedures have been adapted to three-dimensional imaging. Likewise, efforts have been made to improve reconstruction speed and efficiency, for example, through the use of improved reconstruction algorithms. Nevertheless, three-dimensional imaging remains relatively slow.
Echo planar imaging (EPI) is a known imaging technique in which a series of echoes is rapidly induced following a single RF pulse. More specifically, an RF pulse and a slice select gradient are applied to excite resonance in a selected slice and a phase encode gradient is applied to phase encode the resonance. A series of frequency encode or read gradients of alternating polarity is applied in successive fashion. During each read gradient, a magnetic resonance signal or echo is read out. Between each read gradient, a short pulse or blip along the phase encode gradient axis is applied to increment the phase encoding of the resonance by a line in the selected slice. A one-dimensional inverse Fourier transform of each echo provides a projection of the spin distribution along the read axis. A second inverse Fourier transform along the phase encoded echoes provides a second dimension of spatial encoding. Typically, the phase encode gradient blips are selected of an appropriate magnitude that data for a complete field of view is taken following each RF pulse. The total sampling time is determined by the number of sampled points per read gradient and the number of phase encode gradient steps.
Echo volume imaging extends echo planar imaging techniques to multiple planes. After performing the above-described echo planar imaging sequence, a pulse or blip along a secondary phase encoding axis is applied. Typically, the secondary phase encoding blips step the phase encoding along an axis perpendicular to the primary phase encode and read axes. Thereafter, phase encode gradient blips are applied between each read gradient to step line by line in the primary phase encode direction. Because the phase encode blips in the first k-space plane move the phase encoding to one extreme edge of the field of view, the phase encoding blips in the second k-space plane in the secondary phase encode direction are typically of the opposite polarity to step the phase encoding back in the opposite direction. In this manner, the multiple planes are aligned, but offset in steps in the z-direction. One disadvantage of the above echo planar imaging and echo volume imaging techniques is that the trajectory through k-space is reversed in time for alternate phase encode lines or views. This causes phase discontinuities which can result in ghosting.
Spiral echo planar imaging techniques are also known, in which the applied x- and y-gradient pulses, i.e., along the traditional read and phase encode axes, are sinusoidally varying and linearly increasing. In this manner, data sampling commences at the center of the field of view and spirals outward, covering the field of view along a spiral k-space trajectory. One of the drawbacks of spiral echo planar imaging, however, is that it is a single slice technique. To obtain multiple slices, the spiral echo planar imaging technique is repeated multiple times. An RF excitation pulse and slice select gradient followed by sinusoidally varying and linearly increasing x and y-gradients are applied for each slice to achieve coverage of the volume of interest.
Accordingly, the present invention contemplates a new and improved magnetic resonance imaging apparatus and method wherein three-dimensional or depth information is acquired following a single RF excitation pulse.
In accordance with the present invention, a magnetic resonance imaging technique is provided in which data is collected in a selected number of planes or partial planes of k-space.
In accordance with one aspect, a method of diagnostic imaging comprises establishing a polarizing magnetic field in an examination region and applying a radio frequency pulse to excite magnetic resonance in a volume of interest as a whole. After exciting magnetic resonance in the whole volume, the excited magnetic resonance is simultaneously sampled and spatially encoded in the absence of radio frequency pulses to collect data for a plurality of intersecting planes or partial planes of k-space data. Each plane of k-space data corresponds to a different view through the volume of interest. The plurality of planes or partial planes of k-space data are collected without collecting a complete three-dimensional k-space data set.
In accordance with a further aspect, a magnetic resonance imaging apparatus includes a magnet system for creating a temporally constant magnetic field through an examination region in which at least a portion of an object to be imaged is placed and a radio frequency excitation system for applying radio frequency excitation to a volume of interest of the object to be imaged. A receiver system detects and demodulates magnetic resonance data from the volume of interest and a magnetic field encoding system applies encoding magnetic fields to provide spatial discrimination of magnetic resonance data from the volume of interest within a single radio frequency excitation period. The magnetic field encoding system spatially encodes the magnetic resonance signal data along a preselected k-space trajectory, the k-space trajectory covering a plurality of intersecting planes or partial planes of k-space data.
One advantage of the present technique is that it provides the speed characteristics of echo planar imaging while providing a plurality of views which can be used for depth perception purposes.
Another advantage of the present invention is that three dimensions can be spatially encoded in a single shot, with no noticeable delay between the different views, thus eliminating inconsistencies, blurring, or other artifacts due to many types of motion, such as patient motion, physiological motion, and so forth.
Another advantage is that the present invention can be readily adapted to applications using a short data collection window, such as freezing the motion of rapidly moving structures, following a bolus injection of contrast media, and so forth.
Another advantage is that the present technique can be readily adapted to interventional methods, such as needle or catheter tracking.
Still another advantage is that the present invention can be used to provide rapid depth cues for interactive imaging applications.
Another advantage is that sampling is more efficient because the full volume of k-space data is not collected.
Another advantage of the present invention is that it decreases patient examination time.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.